Studies of transcriptional regulation of inherited viral genes provides an approach to learn how cells control expression of various genes involved in cancer. Murine mammary tumor virus expression in both cell culture and in whole animals has been used to study the genetic and non-genetic influences governing transcriptional regulation. Glucocorticoids stimulate the rate of viral RNA synthesis in cell culture. To investigate this process further, attempts have been made to utilize various methods for detecting newly synthesized RNA in cell-free systems. We concluded that further understanding of transcriptional regulation requires a detailed understanding of the primary structure of the viral nucleic acid. The relevance of regulation of mammary tumor virus expression to natural mammary adenocarcinoma in mice was shown by the high correlation between MMTV milk virus titer and the propensity to develop tumors. In multiple second backcross females between high and low incidence mouse strains, simple Mendelian genetic influences could not be demonstrated because of the multifactorial etiology of the mammary tumors. BIBLIOGRAPHIC REFERENCES: Young, H.A., Shih, T.Y., Scolnick, E.M. and Parks, W.P.: Steroid induction of mouse mammary tumor virus: Effect upon synthesis and degradation of viral RNA. J. Virol. 21: 139-146, 1977. Troxler, D.H., Parks, W.P., Vass, W.C., and Scolnick, E.M.: Isolation of a fibroblast nonproducer cell line containing the Friend strain of the spleen focus forming virus. Virology 76: 602-615, 1977.